A primary objective of electronics today is the miniaturization of equipment while maximizing performance per unit price. This objective is especially important in electronic communications equipment where size and weight are a concern. Cellular telephones and radios for aircraft and land vehicles are but a few examples of devices wherein miniaturization is a major objective. An important component in many communications devices is a monolithic filter that allows a desired signal at a specific frequency to pass into the electronic device while filtering out unwanted signals.
FIGS. 1 and 2 illustrate a standard two-pole monolithic crystal filter package containing a crystal blank, a base upon which the crystal blank is mounted and a can or cover for enclosing the crystal blank. As should be understood, the crystal blank is relatively large and round and resembles the shape of a circular-like disk. The crystal disk is mounted in a substantially perpendicular orientation relative to the base so that the resulting height dimension of the filter package is greater than the diameter of the disk. Input and output electrodes are formed near the center of one surface of the disk. Ground electrodes are formed on an opposite surface of the disk and are disposed in substantial registry with the input and output electrodes. Ground and mounting contacts are formed at specific locations along the perimeter of the disk. The electrodes are electrically connected to the contacts by thin "flags" formed on the surface of the disk. Commercially available monolithic crystal disk filters are available in multiple pole designs, such as two-pole, four-pole, six-pole, eight-pole, etc.
Many factors must be taken into account when designing a monolithic filter having a specific output signal performance. Monolithic filters are typically designed to have an output signal with a specific center frequency and bandwidth. The filters are also designed to produce an output signal with predetermined levels of ultimate attenuation, insertion loss (loss of output signal strength) and spurious responses (unwanted output signal responses). For example, electrical devices operating on a principle of amplitude modulation (AM) require a narrower bandwidth than devices operating on a principle of frequency modulation (FM). Similarly, devices operating on FM require a narrower bandwidth than devices operating on pulse modulation (PM).
Bandwidth or bandpass is a function of electrode separation, length of electrode and thickness of the crystal blank and plated elements. The closer the electrodes are together, the wider the bandwidth of the output signal. The thicker the crystal blank, the lower the center frequency and the wider the bandwidth of the output signal. Insertion losses are typically greater for output signals with narrow bandwidths due to the increase in Q of the circuit. Spurious responses are typically more active for output signals with wide bandwidths. A more complete discussion of crystal filter design can be found in the book entitled Crystal Filters--Design, Manufacture, and Application authored by Robert G. Kinsman and published by John Wiley & Sons, Inc. in 1987, the substance of which is incorporated by reference herein.
The size and shape of the crystal blank are important limitations when designing a miniaturized monolithic crystal filter. The closer the electrodes are to the contacts or the edges of the blank, the greater the insertion losses and spurious responses will typically be. In standard crystal filter designs, the electrodes are typically no closer to the edge of the blank than ten times the thickness of the blank. As noted above, the output signal bandwidth is also a function of the distance between the input and output electrodes. The electrode pairs must be spaced apart the most for low frequency, narrow bandwidth filters. The size of the electrodes must also be taken into consideration. The larger the area of the electrodes, typically the lower the center frequency of the output signal. The longer the electrodes, typically the narrower the bandwidth.
One problem with commercially available monolithic filter designs is the size and shape of the crystal disk. The large size and round shape of the disk contained in conventional crystal filters is detrimental to the miniaturization of electronic devices. This specific problem has become even more pronounced in recent years given the trend in the electronics industry to split a single circuit board containing a large number of components into several separate circuit boards containing a portion of the components. These separate circuit boards are then stacked, one on top of the other, to effectuate the miniaturization of the electronics devices. As should be appreciated, the distance between the circuit boards is often dictated by the height of the monolithic crystal filter.
Many attempts have been made to reduce the height of monolithic filters. More specifically, the electronics industry has attempted to install monolithic crystal disk filters in ceramic chip carriers. However, even after substantial research and capital expenditures, the amount of circuit board space or real estate consumed by the ceramic chip carriers continues to be too large. As should be understood, circuit board real estate is the surface area of the circuit board consumed by a given component, such as the monolithic filter and similar assemblies. The combined cost of the ceramic chip carrier and the increase in real estate consumed by the ceramic chip carrier more than offsets the costs and benefits obtained by the reduction in height for commercial production purposes.
The electronics industry also attempted to substitute monolithic crystal disk filters with surface acoustic wave (SAW) filters. However, SAW filters are proving to be unsuitable for many purposes. For example, the center frequency of a standard SAW filter drifts with changes in temperature. These filters are generally used in applications experiencing a narrow temperature range of between 20.degree. C. to 50.degree. C. Commercial applications typically require filter output stability between a larger range of temperature -30.degree. C. to 70.degree. C., and military applications require a temperature stability between -40.degree. C. to 85.degree. C.
The electronics industry has also attempted to reduce the height of monolithic crystal disk filter packages themselves. As best shown in FIGS. 1 and 2, these filter packages have input and output posts or leads that project from the lower surface of the base. These posts can be bent so that both the crystal disk and the filter package lay flat against or parallel to the circuit board. Unfortunately, these same designs are not readily mountable to circuit boards, especially by conventional surface mount pick-and-place machines which are widely employed in this industry segment. Other designs have included a crystal disk mounted in a substantially parallel orientation relative to the filter base. An example of such a design is shown in U.S. Pat. No. 5,281,935, the disclosure of which is incorporated by reference herein. The most noteworthy problem associated with these designs, however, is that the monolithic crystal disk filter packages take up a great deal of circuit board real estate when the large crystal disk is positioned in a substantially parallel orientation relative to the circuit board. Again, the costs and associate benefits of the reduction in filter height is viewed as being offset by the costs attendant to the increase in real estate consumed by the individual filters.
Other attempts in the art were made to modify commercially available end mounted, discrete crystal strip resonators illustrated in FIGS. 3 and 4 to produce an end mounted, monolithic crystal strip filter shown in FIGS. 5 and 6. Although commercially unsuccessful, these end mounted or tubular monolithic filters would have reduce the real estate consumed by the base of the filter when mounted in an upright orientation on the circuit board. Unfortunately, while commercially acceptable discrete crystal strip resonators are available in both end mounted designs similar to that shown in FIGS. 3 and 4 and low profile designs similar to that illustrated in FIG. 7, to date, no monolithic crystal strip filter exists in either an end mounted or low profile designs.
Another problem encountered with miniaturized monolithic crystal filters is the amount of spurious responses and insertion loses which are associated with the output signal. One reason for increases in spurious responses and insertion loses is believed to be the close placement of the electrodes relative to the mounting and ground contacts as a result of the reduced size of the crystal blank. As should be understood, small crystal blanks simply do not provide a great deal of surface area within which to position various components when designing specific monolithic filters which are intended for particular applications.
A further problem faced by the electronics industry is maintaining a competitive price for miniaturized crystal filters. This requires particular attention to the costs associated with manufacturing the individual miniaturized filters. To minimize manufacturing expenses, monolithic crystal filter designs should utilize standard bases, such as HC-45, HC-49 or UM-1 base designs. The miniaturized filter should further be mountable on the base without extensive modification of the base or to the machines employed to manufacture the base. Filter designs that require substantial retooling of these manufacturing machines prior to commercial production result in dramatic increases in the cost of the miniaturized filters which normally renders the filters impractical for commercial purposes.
A still further problem faced by the electronics industry is that miniaturized crystal filters should be adapted for installation on existing circuit boards without significant redesign of the component layout of the circuit boards. As would be expected, modifications to the existing circuit board designs can dramatically increase the cost of employing the miniaturized filter, and thus the cost of the electronic devices utilizing the filter.
The present invention is provided to solve these and other problems.